Amblyopia is defined as poor vision in a structurally sound eye, and with a prevalence of 3-5%, it is the leading cause of vision loss in childhood. Amblyopia results from the inability of the brain to correctly interpret visual input due to deprivation or suppression. Anatomical risk factors for this condition include strabismus, anisometropia, cataract, certain forms of astigmatism, and hyperopia. Early detection and treatment is essential to prevent irreversible vision loss, but the risk factors can be difficult to detect. While comprehensive eye exams have been mandated in some areas, in most cases this solution is not economically feasible and tends to be instituted later than is optimal for amblyopia detection. Ideally, all children would be screened for amblyopic risk factors before age 4 or 5.
Practical vision screeners with sufficiently high testability, sensitivity, cost effectiveness, speed and specificity to reliably identify children at risk for amblyopia have been difficult to implement. Visual acuity tests have been the most widely used approach to vision screening. However, visual acuity testing may be no better than other screening tests for detecting amblyopia.
Guyton, Hunter, et. al., in U.S. Pat. No. 6,027,216 (hereby incorporated by reference) disclose a method of eye fixation monitoring using retinal reflections of polarized light to determine foveal fixation. This system was designed to detect both ocular focus and alignment. The object of the PVS is to provide a first-stage screening device that will differentiate between children in need of referral to an ophthalmologist and those not at risk, without attempting diagnosis. The output of the device is binary (either “refer” or “pass”) to facilitate use by non-ophthalmologists.